The present invention relates to a waveform generator circuit which generates a waveform with digital circuitry, and more particularly to an electronic musical instrument in which the rate of accessing a waveform changes in one cycle of the waveform.
With the progress of digital technology, it has become possible to generate waveform data by means of digital circuitry and to convert the digital waveform data into an analog signal by means of a digital-to-analog converter, thereby to produce an analog signal waveform. Such waveform generation by digital circuitry is also applied to electronic musical instruments, and the products of electronic musical instruments capable of generating waveforms of various tone colors are implemented.
Until now, the musical sound generating systems of the electronic musical instruments based on digital circuitry as stated above have included (i) a sinusoidal wave synthesis system, (ii) a variable filter system, (iii) a waveform memory readout system, (iv) a frequency modulation system, etc.
The sinusoidal wave synthesis system (i) is a system wherein the sinusoidal wave signals of a fundamental wave and higher harmonics are generated by a digital circuit, and these digital waveform signals are synthesized to produce a musical sound of desired tone color. In case of producing musical sounds in desired harmonic overtone forms, this system needs computing channels which are equal in number to the sorts of required harmonic overtones. Further, in case of changing a spectrum with time, higher harmonics control signals equal in number to the sorts of harmonic overtones are needed, for varying amplitude levels for the respective harmonic overtones. This system has the problems that the generator circuit becomes large in size because the aforementioned computing channels and higher harmonics control signals necessitate circuits equal in number to the sorts of harmonic overtones, and that the generation control of the higher harmonic control signals becomes complicated.
The variable filter system (ii) is a system wherein a digital filter is used, and the frequency characteristic of the filter is changed by a variable signal. This system has the problem that the circuit of the digital filter becomes large in size. Further, in a case where a waveform is generated at a fixed sampling rate, that is, where the fundamental tone to be inputted to the digital filter is generated at a fixed sampling rate, a waveform having a large number of higher harmonics is difficult to obtain, resulting in the problem that the effect of the digital filter in a higher harmonics region decreases to half. This system also has the problem that folded distortion arises.
The waveform memory readout system (iii) is a system wherein waveform data stored in a memory or the like in advance is sequentially read out in correspondence with a phase angle, thereby to generate a waveform. Since the aforementioned waveform data stored in the waveform memory is the data of a musical sound waveform to be produced as a musical sound, the spectrum of the waveform has been fixed. In order to change the spectrum, therefore, waveform data corresponding to the change of the spectrum must be stored in the memory, and moreover, a control circuit for reading out the data successively in correspondence with the change of the spectrum is needed. This system accordingly has the problems that the capacity of the memory is large and that the control circuit is complicated.
The system (iv) is an application of frequency modulation, and is a system wherein, using the two sinusoidal waves of a carrier wave and a modulating wave, the frequency ratio and the modulation depth are changed thereby to change a harmonic overtone. This system can control the harmonic overtone to some extent. Since, however, each harmonic overtone changes according to a Bessel function, it has been difficult to obtain a musical sound whose spectrum has a smoothly changing envelope, for example, whose amplitude value decreases as the waveform changes from the fundamental wave toward the higher harmonics.
Further, there is a system wherein a peak (hereinbelow, termed the "formant peak") is possessed in the higher frequency region of the spectrum of a musical sound waveform, and the formant peak frequency is changed with time, thereby to bestow a change on a musical sound. An example is to utilize the resonance effect of a voltage control filter VCF in an analog synthesizer. Methods of generating the aforementioned formant peak by means of a digital circuit include (a) a method wherein the coefficient of a harmonic overtone synthesized by adding sinusoidal waves is changed with time so as to give rise to a filter effect, and to generate a peak value in the amplitude values of higher harmonics of higher orders, and (b) a method wherein a resonance effect as attained with an analog filter is produced by a digital low-pass filter. The method (a) is the same as the foregoing system (i). It requires computing channels corresponding to the higher-order frequencies in order to generate the higher harmonics, and besides, it needs to set amplitudes for the respective higher harmonics, namely, harmonic overtones, so that a complicated circuit is necessitated and has been difficult to fabricate. With the method (b), the circuit of the digital filter becomes larger in size and has similarly been difficult in realization.